Filter elements normally require a very large surface area to permit a free flow of fluid therethrough. In addition, filter elements have a further requirement to permit the accumulation of a large quantity of foreign matter therein without restricting the flow of fluid excessively therethrough.
The normal range of pore sizes in a filter are such that foreign particles larger than a certain size cannot pass through the filter. When flaws, such as a ruptured element are present in a filter element, particles larger than the normal pore size range can pass through the filter and render the filter ineffective for its intended use. Thus, even a small rupture in a filter element is considered a failure and must be detected. Various prior art devices and methods have been devised to test filter elements for such flaws.
U.S. Pat. No. 1,395,247 to A. Abrams, issued Nov. 1, 1921, discloses an apparatus for generating hydrogen sulphide smoke and for applying this smoke to test the porosity of materials. Abrams determines the distribution of pores in the test material by first saturating the filter element with a litmus indicator. Abrams then passes the hydrogen sulphide smoke through the filter element and when the hydrogen sulphide passes through the filter, an indication will be shown on the litmus paper to show how the pores are distributed in the filter element. This test, however, is time consuming, complicated and could significantly reduce the lift of the filter element tested. In addition, Abrams' test method could pose a health hazard to the operator.
Finkelstein, in U.S. Pat. No. 2,072,872, issued Mar. 9, 1937, discloses an apparatus for testing filters utilizing smoke generated by directing atomized oil against an electrically heated plate. This smoke is then passed through the filter element being tested. The smoke passing through the filter element is measured by a light sensitive cell. This apparatus is also complicated and time consuming. In addition, it has been found that smoke particles generated by hydrocarbon oils have a tendency to agglomerate and, thus, there may be a wide variation in the size of smoke particles generated by this device. This can cause erratic, unrepeatable test results.
Another prior art filter testing device is disclosed in U.S. Pat. No. 2,819,608 issued to McLaren et al on Jan. 14, 1958, owned by the assignee of the present patent application. McLaren et al discloses an apparatus for testing the performance of filter elements by utilizing water and spherical glass beads having a known size distribution. A first beam of light is projected through the fluid before it passes through the filter element and a second beam of light is projected through the fluid after it passes through the filter element. The ratio of the amount of light scattered in the first and second beams provides an instantaneous indication of the relative density of the contaminant before and after passing through the filter element. Thus, McLaren's device provides a continuous measurement of the efficiency of the filter. However, this apparatus and method is also time consuming and may be harmful to the life of certain filters.
In U.S. Pat. No. 2,833,140 issued to A. E. W. Austen et al, on May 8, 1958, a filter testing apparatus utilizing air entrained dust is disclosed. Austen provides a chamber for accommodating a filter to be tested and then introduces air entrained dust into the chamber. Austen then provides a baffle means for promoting the deposition of the airborne dust and an inspectable surface on which such deposition takes place. This apparatus is also time consuming requiring a visual inspection of each filter element after the test. Therefore, the apparatus is not suitable for testing large quantities of filters as usually required for quality control purposes.
Tuttle, in U.S. Pat. No. 3,336,793 issued Aug. 22, 1967, discloses a filter test mechanism apparatus wherein a filter element is chucked on a rotatable spindle which is a piston rod of a fluid cylinder. Retraction of the piston rod by the piston deforms an elastic ring to hold the element firmly and to seal the open ends of the filter element. The filter is then immersed and rotated within a liquid bath to entirely coat the filtering area of the filter element. Then, pressure regulated gas is admitted into the interior of the filter. The interior of the filter element and a fluid pressure indicating instrument thus forms a closed system except for the pores of the filter element medium and any flaws present therein. Tuttle then measures the drop in pressure of this closed system so as to indicate leakage of the filter element. This device and mechanism is also not suitable for all types of media and may cause contamination of the filter element in certain applications. In addition, the method and apparatus is time consuming and not suitable for use in testing large numbers of filter elements.
Another apparatus and method for determining structural failure of filter elements is disclosed by A. J. Taylor et al in U.S. Pat. No. 3,608,354, issued Sept. 28, 1971. Taylor determines a failure in a filter element by sensing a drop in the pressure of fluid flowing through the filter. To accomplish this, a support is provided for a filter element and contaminated fluid is pumped from a reservoir to the filter element by means of a pump through a supply conduit. A drain conduit returns the fluid from the filter to the reservoir. The pressure of the fluid before entering the filter is indicated on a suitable pressure gage. A drop in pressure in the supply conduit is sensed by a differential pressure sensing device having opposite sides connected to the supply conduit near the filter element support. One side of the sensing device is connected directly by conduit to the supply conduit while the other side is connected to the supply conduit by means of a second conduit which has unidirectional flow means interposed in the conduit. The unidirectional flow means permits the free flow of fluid in the direction towards the sensing device but resists fluid flow in the opposite direction. When a rupture in a filter element under test occurs, a reversal in pressure creates a differential pressure across the pressure switch which causes the pump motor to stop and activates a solenoid of a bypass valve to open. Thus, the differential pressure switch indicates a rupture of the filter element. While this device is purported to be automatic in operation, it is time consuming and not suitable for testing large numbers of filter elements. In addition, this device may cause contamination of the filter element for certain types of filter media.
In summary, therefore, none of the known prior art devices provide an apparatus and method which is suitable for all types of filter media, which does not significantly reduce the life of the filter media and is suitable for testing large numbers of filters elements.